Compact electric circuit breakers utilizing movable contact arms operated by an operating mechanism in an insulated housing are employed in a variety of residential, commercial and industrial applications. The over-current sensing trip units for such devices comprise thermal-magnetic trip units typically employed in the high volume, low cost markets. Heating of the thermal element in the thermal-magnetic trip system varies inversely with the square of the current, which results in the use of directly heated bimetals for low ampere rated devices, and indirectly heated bimetals for higher ampere rated circuit breakers. In an indirectly heated bimetal, the current passes through an adjacent heater element which "indirectly" heats the bimetal strip by thermal conduction, convection and radiation. In a directly heated bimetal, I.sup.2 R heating occurs by current passage through the bimetal strip. In both trip units, the heat causes the bimetal strip to deflect and operate a trip latch which works in cooperation with the operating mechanism. Under short circuit conditions, abnormally high currents that pass through the directly heated bimetal could possible damage the bimetal.
U.S. Pat. No. 3,046,371 entitled "Circuit Breaker" teaches, inter alia, the use of a bimetal shunt that incorporates shunting contacts that are closed upon actuation of the magnet armature. Closing of the shunt contacts produces an alternate current path that is parallel to and bypasses the bimetal, thereby protecting the bimetal from short circuit currents.
U.S. Pat. No. 3,548,358 entitled "Electric Circuit Breaker with Bimetallic Strip Protective Means" provides additional means to break contact welds that occur under intense short circuit conditions and is effective in protecting the bimetal under short circuit conditions. However, in extremely high short circuit conditions, the parallel path configuration of the shunt system still permits excessively high currents to flow through the bimetal.
U.S. Pat. No. 4,749,829 entitled "Circuit Breaker" teaches the use of an in-line current limiting resistor to reduce the level of let-through current experienced by the thermally sensitive trip unit under short circuit conditions. An ohmic resistor is connected in series between the line strap and trip unit, and thermally coupled to the circuit breaker housing. Under short circuit conditions, the additional in-line resistance limits the maximum current that is permitted to pass through the circuit breaker, effectively protecting the bimetal from damage. During quiescent operating conditions, I.sup.2 R heat generated by the ohmic resistor is dissipated by thermal conduction through the circuit breaker housing. The current limiting characteristics of the circuit breaker are enhanced as the resistance value of the resistor increases. However, the amount of resistance that can be incorporated within an ohmic resistor is limited by the overall thermal considerations of the circuit breaker.
U.S. Patent Docket No. 41PR-7406 filed on Feb. 10, 1997 entitled "Circuit Breaker Current Limiting Arc Runner" teaches the use of a current limiting arc runner that does not conduct circuit current under quiescent operating conditions, but does conduct arcing current under overcurrent conditions by commutating the arc to an arc rail whereby additional I.sup.2 R thermal heating is eliminated. Since the current limiting arc runner is electrically connected in series with the trip unit during short circuit arcing conditions, the bimetal could be subjected to high levels of instantaneous peak currents.
One purpose of the invention is to provide an efficient bimetal shunting system that completely switches the current to an alternate path upon the occurrence of arcing to bypass the thermally sensitive bimetal during a short circuit overcurrent condition. Another purpose of the invention is to incorporate a current limiting element which does not generate additional I.sup.2 R heating during quiescent operating conditions. Yet another purpose of the invention is to incorporate a current limiting element which has superior current suppressing characteristics for effective current limitation during extremely high short circuit over-current conditions.